Page 79 - Synthetic Fuels Handbook

66 CHAPTER THREE

3.2 PETROLEUM RECOVERY

Petroleum is recovered from reservoir by various methods, the most advantageous of which
is the use of the inherent reservoir energy that allows the petroleum to flow to the surface
(Speight, 2007). However, the production rates from reservoirs depend on a number of fac-
tors, such as reservoir pressure, rock type, and permeability, fluid saturations and proper-
ties, extent of fracturing, number of wells, and their locations.
Primary oil recovery depends on the natural energy contained in the reservoir to drive
the oil through the complex pore network to producing wells. The driving energy may
come from liquid expansion and evolution of gas dissolved in the oil as reservoir pres-
sure is lowered during production, expansion of free gas in a gas “cap,” influx of natural
water from an aquifer, or combinations of these effects. The recovery efficiency for primary
production is generally low when liquid expansion and solution gas evolution are the driv-
ing mechanisms. Higher recoveries are associated with reservoirs having water or gas cap
drives and from reservoirs where gravity effectively promotes drainage of the oil from the
pores. Eventually, the natural drive energy is dissipated. When this occurs, energy must be
supplied to the reservoir to produce additional oil.
Secondary oil recovery involves introducing energy into a reservoir by injecting gas or
water under pressure. The injected fluids maintain reservoir pressure and displace a portion
of the remaining crude oil to production wells.
Waterflooding is the principal secondary recovery method and currently accounts for
almost half of the U.S. daily oil production. Limited use is made of gas injection because
of the value of the natural gas. However, when gravity drainage is effective, pressure main-
tenance by gas injection can be very efficient. Certain reservoir systems, such as those with
very viscous oils and low permeability or geologically complex reservoirs, respond poorly
to conventional secondary recovery techniques. In these reservoirs improved geologic
understanding and use of enhanced oil recovery (EOR) operations should be employed as
early as possible.
Conventional primary and secondary recovery processes, at existing levels of field
development, will ultimately produce about one-third of the original oil in place (OOIP)
in discovered reservoirs. For individual reservoirs the recovery ranges from the extremes
of less than 5 percent to as much as 80 percent of the OOIP. The range chiefly reflects the
degree of reservoir complexity or heterogeneity. The more complex the reservoir, the lower
the achievable recovery.
Of the remaining two-thirds of original oil in place in domestic reservoirs, a portion of
this oil can be recovered through advanced secondary recovery methods involving improved
sweep efficiency in poorly swept zones of the reservoirs. For these reservoirs, well place-
ment and completion techniques need to be pursued consistent with the degree of reservoir
heterogeneity. Such improved secondary oil recovery can be accomplished using advanced
geologic models of complex reservoirs.
The balance of the remaining two-thirds of unrecovered oil is oil that is or will be
residual to efficient sweep by secondary recovery processes. Portions of this residual oil
can be recovered by tertiary or enhanced oil recovery methods. The intent of enhanced
oil recovery methods is to increase ultimate oil production beyond that achieved by pri-
mary and secondary recovery methods by increasing the volume of rock contacted by the
injected fluids (improving the sweep efficiency), reducing the residual oil remaining in
the swept zones (increasing the displacement efficiency), or by reducing the viscosity of
heavier oils.
Current enhanced oil recovery methods can be broadly grouped into three categories:
(a) thermal methods, (b) miscible methods, and (c) chemical methods (Speight, 2007 and
references cited therein). These processes differ considerably in complexity, the physical
mechanisms responsible for oil recovery, and maturity of the technology derived from field